<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      width: 300px;
      height: 200px;
      /* background-image: linear-gradient(
        pink,
        green,
        red,
        orange
      ); */
      background-image: linear-gradient(
        transparent,
        rgba(0, 0, 0, .6)
      );
    }
  </style>
</head>
<body>
  <div class="box"></div>
</body>
</html>